1. Field of the Invention
The present invention relates to a magnetic recording medium and to the use of a non-aqueous suspension of inorganic solid particles in the production of the same. More particularly, the present invention relates to a magnetic recording medium comprising a magnetic layer and/or a back coat layer in which inorganic solid particles are well dispersed so that electromagnetic conversion characteristics, traveling stability and durability of the magnetic layer and abrasion resistance and surface smoothness of the back coat layer are improved, and to a non-aqueous suspension of inorganic solid particles suitable for use in the formation of the magnetic layer and/or back coat layer of the magnetic recording medium.
2. Description of the Prior Art
A magnetic recording medium is generally produced by coating a magnetic paint comprising a magnetic powder, a binder resin, a solvent and other necessary additives on one or both surfaces of a substrate such as a polyester film and drying it to form a magnetic layer on the substrate.
The magnetic recording medium is required to have good electromagnetic conversion characteristics, traveling stability and durability. To improve the traveling stability and durability of the magnetic layer, it is proposed and practiced to add relatively hard non-magnetic solid particles, such as Al.sub.2 O.sub.3 powder, and TiO.sub.2 powder to the magnetic layer.
The magnetic recording medium to be used as a video tape has a back coat layer in order to prevent non-uniform winding during high speed winding and electrostatic charge and to improve traveling stability and durability of the medium. To prevent electrostatic charge and to improve the traveling stability and durability, the back coat layer is formed by coating a mixture of carbon black and suitable solid additive particles as well as a bider resin on the back surface of substrate.
Generally, the non-aqueous suspension of inorganic solid particles to be used for the formation of the magnetic layer or back coat layer comprises the non-magnetic particles, a non-aqueous solvent and a dispersant. In such a suspension, it is required for the inorganic solid particles to be divided to substantially their primary particles and to keep such a good dispersed state for a long time. However, since surfaces of most kinds of the inorganic solid particles are hydrophilic, it is very difficult to homogeneously and stably disperse the inorganic solid particles in the non-aqueous lipophilic solvent. Therefore, it is proposed to treat the surfaces of such inorganic solid particles with a suitable dispersant. For example, fatty acids, titanate type coupling agents and aluminum coupling agents are suggested as dispersants for treating the surfaces of inorganic solid particles (cf. Japanese Patent Kokai Publication Nos. 170026/1985 and 129927/1985).
When the surfaces of inorganic solid particles are treated with the dispersant, the dispersibility of the particles and stability of the dispersion are improved, but their improvement is still not satisfactory. If such a conventional dispersant is used for treating the inorganic solid particles, the treated solid particles still have an insufficient affinity with the binder resin component in the magnetic layer and less dispersibility in the magnetic layer, so that the traveling stability and durability of the magnetic recording medium are not desirably improved. In addition, the surface evenness of the magnetic layer is deteriorated and in turn the electromagnetic conversion characteristics are decreased. Further, in the case of the back coat layer, its surface evenness is deteriorated and abrasion resistance is insufficient. Therefore, the traveling property of the magnetic recording medium is not improved.